Sagan & Swan’s Voyager Mars Landing Sites (1965)

Until the 1980s, most U.S. automated space explorers bore names connoting ventures into unknown parts – Explorer, Pioneer, Ranger, Surveyor, Mariner, and Voyager. Most people today identify the last of these names with the spectacularly successful pair of outer Solar System flyby spacecraft launched in the late 1970s. There was, however, an earlier Voyager program. First proposed in 1960 as a follow-on to the planned Mariner planetary flyby program, the original Voyager aimed to explore Venus and (especially) Mars using orbiters and landing capsules.

Carl Sagan, an assistant professor of astronomy at Harvard, and Paul Swan, Senior Project Scientist at Avco Corporation, published results of a study of possible Voyager Mars landing sites in the January-February 1965 issue of Journal of Spacecraft and Rockets. For their study, they invoked a Voyager design Avco had developed in 1963 on contract to NASA Headquarters. The “split-payload” design comprised an orbiter “bus” based on the Jet Propulsion Laboratory’s Mariner (or proposed advanced Mariner-B) design and a landing capsule shaped like the Apollo Command Module (that is, conical, with a bowl-shaped heat shield). Bus and capsule would leave Earth together on a Saturn IB rocket with an “S-VI” upper stage (a modified Centaur stage).

Avco’s 1963 Voyager design. Image: NASA

The Voyager lander would be sterilized to prevent biological contamination of Mars. Near Mars it would separate from the orbiter, enter the martian atmosphere, and float to a gentle touchdown suspended from a parachute. The Avco design included no landing rockets, which meant that more lander mass could be devoted to instruments for exploring the planet. The lander would operate on Mars for at least 180 days. The Voyager orbiter, meanwhile, would fire rockets to slow down so that Mars’s gravity could capture it into a polar orbit, from which it would image the entire martian surface and serve as a radio relay for the lander.

Swan and Sagan noted that operational constraints would limit possible Mars landing sites. For example, the orbiter and Earth would need to rise at least 10° above the horizon at the landing site to permit daily radio communication sessions, and the Sun would need to be rise at least 10° above the horizon so that the lander’s solar-powered science instruments could function properly. Such constraints would combine to create landing “footprints” that would vary widely depending on the Earth-Mars transfer opportunity used. The footprint for the 1969 minimum-energy opportunity, for example, would take the form of a north-pointing wedge centered on 270° longitude and spanning from 70° south to 60° north latitude.

Avco’s Voyager lander was designed so that it could be targeted to specific regions within such footprints, Sagan and Swan noted. They proposed that exobiologically interesting sites be accorded top priority in Voyager lander site selection. Sagan and Swan then looked at possible exobiologically interesting areas accessible to the Voyager landers launched during the 1969, 1971, 1973, and 1975 minimum-energy opportunities.

The U.S. Air Force Aeronautical Chart and Information Center based its MEC-1 prototype Mars map on data current as of 1962. This is the Mars Sagan & Swan knew when they planned their Voyager landing sites. Image: U.S. Air Force/Lunar and Planetary Institute

Their list of such sites was, of course, based entirely on Earth-based telescopic observations, for no spacecraft had yet visited Mars. They also used surface feature names that had been assigned by telescopic observers (image at top of post); those names would be superseded soon after the 1971-1972 Mariner 9 Mars orbiter mission.Sagan and Swan described the “wave of darkening” observed since the 19th century. The “wave” was regularly observed spreading from the pole to the equator in the martian springtime hemisphere. When they wrote their paper, it was widely interpreted as indicative of martian water, atmospheric circulation, and vegetation. Theory had it that, as the polar ice cap melted, atmospheric moisture increased and circulated toward the equator. Hardy plants then darkened as they absorbed the moisture from the thin air.

The first two Voyager landers would reach Mars on 31 October 1969, during springtime in the planet’s southern hemisphere. The wave of darkening would be near its peak, making it the best biological exploration opportunity until 1984. Top priority landing sites would include the northern hemisphere regions Solis Lacus and Syrtis Major, which Sagan and Swan described as the “[d]arkest of the Martian dark areas.” On the landing date, both regions would lie at the northern extreme of the southern hemisphere darkening wave and would be relatively warm.

Voyager spacecraft launched in the 1971 minimum-energy opportunity would arrive at the planet on 14 December 1971. Swan and Sagan noted that the 1971 opportunity would need the least amount of energy of any opportunity they considered, and suggested two possible ways of taking advantage of this. Four landers (two per orbiter) could reach Mars as the southern hemisphere wave of darkening faded. Top priority landing sites for this approach would be the southern polar cap, southern hemisphere dark areas Mare Cimmerium and Aurorae Sinus, and Lunae Palus in the north.

Alternately, the 1971 Voyager missions could use a higher-energy path to deliver two landers to Mars as the southern hemisphere darkening wave began. “Thus,” they wrote, “the exobiologically highly desirable characteristics of the 1969 arrival [could] be completely duplicated in the 1971 launch period.”

In the 1973 opportunity, which would see a landing on 24 February 1974, two landers would explore Mars’s deserts and “the so-called canal features.” The accessible landing sites would be relatively cold on the arrival date. Top-priority sites would include Propontis, a region containing a “typical Martian canal,” and Elysium, a “near circular anomalous bright region of ‘pinkish’ coloration” in the northern hemisphere.

Mariner IV captured image frame 11E at a distance of 12,600 kilometers from Mars on 15 July 1965. The largest crater in the frame, which is 151 kilometers wide, was named Mariner in honor of the spacecraft. The frame is centered in the region labeled Mare Cimmerium in the MEC-1 map above. Image: NASA

Sagan and Swan proposed that two Voyager landers leave Earth during the 1975 minimum-energy opportunity. They would land on Mars on 28 August 1976. Top-priority sites would include the northern polar cap and Mare Cimmerium, where the wave of darkening would reach its peak as the 1975 landers arrived.

The first successful automated Mars spacecraft, 261-kilogram Mariner IV, departed Cape Kennedy, Florida, on an Atlas-Agena rocket on 28 November 1964, and flew past Mars on 14-15 July 1965, six months after Sagan & Swan’s paper saw print. Mariner IV revealed a cratered, distressingly moon-like Mars with an atmosphere ten times less dense than expected. The 21 grainy images of the planet the little spacecraft beamed to Earth revealed no signs of water or life. The Avco Voyager design Sagan & Swan had invoked for their study would have depended entirely on parachutes to descend to a soft landing; Mariner IV showed that, while parachutes might still be used, heavy landing rockets would also be needed to slow a lander enough for a soft touchdown.

Voyager as envisioned shortly before its cancellation in 1967. Two such spacecraft would have been launched on a single Saturn V rocket. Image: NASA

This new operational constraint contributed to NASA’s October 1965 decision to employ the Saturn V as Voyager’s launcher. At least as important as the new Mars atmosphere data in this decision was, however, the desire to find new tasks for the Saturn V after it had done its part to place a man on the moon. In 1964-1965, at the request of president Lyndon B. Johnson, NASA had begun to plan its post-Apollo future. In January 1965, the Future Programs Task Group, a body appointed by NASA Administrator James Webb, recommended that the post-Apollo NASA program be based on Apollo-Saturn hardware.Accordingly, in August 1965, NASA Headquarters formed the Saturn-Apollo Applications (SAA) Program Office. By mid-1966, SAA planners expected to fly as many as 40 manned missions using Saturn-Apollo hardware beginning in 1968.

At about the same time, NASA began high-level agency-wide studies of Saturn V-launched manned Mars/Venus flyby missions – what Charles Townes, chair of the President’s Science Advisory Committee, dubbed a “manned Voyager” program. The first of these missions was expected to leave Earth for Mars in September 1975.

Despite Sagan & Swan’s endorsement of the Saturn V, the fledgling planetary science community harbored mixed feelings about the decision to launch Voyager spacecraft on the giant rocket. The decision in December 1965 to postpone the first Voyager mission to the 1973 Mars-Earth transfer opportunity reinforced these misgivings. Combined with the post-Mariner IV redesign, the switch to the Saturn V drove the estimated Voyager cost-per-mission past $2 billion. The high cost made the program increasingly vulnerable as NASA funding reached its Apollo-era peak in 1965-1966 and began a speedy decline.

In August 1967, in the wake of the Apollo 1 fire, Congress killed Voyager and manned flyby mission studies and slashed funding for the Apollo Applications Program (AAP), as SAA had become known. The manned flyby program all but disappeared from NASA’s collective memory and AAP shrank rapidly to become the Skylab Program. In October 1970, NASA permanently closed the Saturn V assembly line, which had been on standby since 1968. The last Saturn V to fly launched the Skylab Orbital Workshop in May 1973.

Voyager, for its part, rose again. In fact, one might argue that it rose again twice. In October 1967, NASA officials, citing Soviet planetary ambitions, met with Congressional leaders to propose a new NASA robotic program for the 1970s. In the new plan, which Congress first funded in 1968, Viking replaced Voyager. Like the Avco Voyager, Viking comprised a lander and a Mariner-derived orbiter; unlike Avco’s Voyager, the Viking orbiter was meant to retain its lander until after it had captured into Mars orbit. The Viking Program’s Titan IIIE-Centaur launch vehicle was approximately equivalent to Saturn IB-Centaur in capability. Scientists and engineers began to search for landing sites for the twin Viking landers almost as soon as the program received approval; the earliest Viking landing site candidates appear on a map that apparently dates from December 1970.

Funding shortfalls pushed the Viking launches from 1973 to 1975. Viking 1 left Earth on 20 August 1975 (image at top of post), and Viking 2 followed on 9 September 1975. In July-August 1976, the Viking landers became the first and second spacecraft to land successfully on Mars.

Carl Sagan’s career after 1965 is well documented. He was involved in nearly all subsequent planetary missions, including the twin Vikings and twin Voyagers, and became by the early 1980s arguably the most important science popularizer since Galileo Galilei. His death at age 62 in December 1996 left a void that has not been filled. Paul Swan, for his part, led Avco’s seminal 1966 study of manned Mars surface operations and joined the staff of NASA’s Ames Research Center by 1970. He remained active there until at least the late 1970s.

The Voyagers continue to operate more than 34 years after launch and more than 50 years after the Voyager name was first proposed. Voyager 1 is the most distant human-made object; at this writing it is about 120 Astronomical Units (AUs) out (one AU = the Earth-Sun distance of about 93 million miles). Sunlight needs more than 17 hours to reach Voyager 1. Both Voyagers have entered a poorly understood borderland called the heliosheath; Voyager 1 is widely expected to cross the heliopause and enter interstellar space before 2015.